1. Field of the Invention
The present invention relates generally to oxygen sensors and, more particularly, to a planar oxygen sensor.
2. Description of the Related Art
A type of planar oxygen sensor such as the one described on pages 249 to 258 of "Sensors and Actuators 9" (1986) is known.
That is, this planar oxygen sensor has a sensor portion fabricated on a SiO.sub.2 film formed on a silicon substrate. This sensor portion includes a polyimide frame formed to surround a region of the SiO.sub.2 film, a cathode formed directly on the SiO.sub.2 film generally at the center of the frame, an anode formed directly on the SiO.sub.2 film between the frame and the cathode, an electrolytic solution which fills the interior of the frame and in which the anode is immersed, and a gas permeable membrane which covers the internal electrolytic solution and which is supported by the frame.
This planar oxygen sensor has an anode. However, a three-electrode planar oxygen sensor is known which has a reference electrode and a counter electrode instead of an anode.
In the conventional planar oxygen sensors, as described above, a gap is formed between the gas permeable membrane and the cathode, and a comparatively large amount of electrolytic solution exists in this gap. The following problem is therefore encountered. When a partial pressure of oxygen in the atmosphere changes, oxygen molecules are diffused in the large amount of electrolytic solution through the gas permeable membrane, so that the time taken by oxygen molecules to reach the surface of the cathode and to cause a current change by the reduction on the cathode is comparatively long.
The oxygen sensor having two electrodes, i.e., a cathode and an anode entail the problem of the anode being dissipated as a current flows by the reduction of oxygen. If, for example, the cathode is formed of platinum while the anode is formed of silver and silver chloride, the dissipation of the anode are expressed by the following chemical formulae: EQU Cathode O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.- EQU Anode 4Ag+4Cl.sup.- .fwdarw.4AgCl+4e.sup.-
As is apparent from these formulae, at the anode an irreversible reaction takes place for a change from silver to silver chloride. Ordinarily, the durability of oxygen sensors depends upon the amount of anode material. For this reason, the life of the oxygen sensor is short if the size of the sensor is reduced. Also, in small-size oxygen sensors, a composition change due to evaporation of water in the electrolyte causes a drift of the output current from the oxygen sensor.